Method and system for link adaptation in an orthogonal frequency division multiplexing (OFDM) wireless communication system

ABSTRACT

A method and system for link adaptation in an orthogonal frequency division multiplexing (OFDM) wireless communication system are disclosed. The entire sub-channels are divided into a plurality of groups. A channel quality indicator (CQI) is generated for each group based on channel quality status in each group, and communication parameters are adjusted in accordance with the CQI.

CROSS REFERENCE TO RELATED APPLICATION

This application claims the benefit of U.S. provisional application No.60/600,741 filed Aug. 11, 2004 which is incorporated by reference as iffully set forth.

FIELD OF INVENTION

The present invention is related to an orthogonal frequency divisionmultiplexing (OFDM) wireless communication system. More particularly,the present invention is related to a method and system for linkadaptation in an OFDM wireless communication system.

BACKGROUND

Current wireless communication systems provide broadband services suchas wireless Internet access to subscribers. Those broadband servicesrequire reliable and high-rate communications over multi-path fadingchannels. Orthogonal frequency division multiplexing (OFDM) is one ofthe solutions to mitigate the effects of multi-path fading. Thecombination of multiple-input multiple-output (MIMO) and OFDM(OFDM-MIMO) technologies can bring high bandwidth efficiency for localarea network (LAN) or wide area network (WAN) environments.

For an efficient operation of wireless communication systems, a linkadaptation for communication parameters is required. Link adaptation isan approach for selecting communication parameters, including a codingrate, a modulation scheme, a transmit power or the like, in order tomaximize the throughput.

In the OFDM-MIMO systems, water-pouring power/bit allocation (WP) isstrongly suggested to maximize downlink capacity. In order to determinethe WP schemes properly, not only correlation of sub-channels butcorrelation of sub-channels' power should be known. The transmission ofthis information requires considerable overhead. Accordingly, it isdesirable to have alternate approaches to signaling such information.

SUMMARY

A method and system for link adaptation in an OFDM wirelesscommunication system is provided. The sub-channels are divided into aplurality of groups. A channel quality indicator (CQI) is generated foreach group based on channel quality status in each group ofsub-channels, and communication parameters on each sub-channel areadjusted in accordance with the CQI.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows the correlation |P_(k)| versus k for several typical valuesof α when P=256.

FIG. 2 shows the correlation |P_(k)| versus k for two values of P whenα=0.64.

FIG. 3 shows the correlation γ_(k) versus k for several typical valuesof α when P=256.

FIG. 4 shows the correlation γ_(k) versus k for two values of P whenα=0.64.

FIG. 5 is a flow diagram of a process for adjusting communicationparameters.

FIG. 6 shows generation of CQI_(q) ^((t)) for each group ofsub-channels.

FIG. 7 is a diagram of a system for link adaptation.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter, the following embodiments are explained with reference toIEEE 802.11 system. However, it should be noted that the embodiments arenot limited to the IEEE 802.11 system, but may be applicable to anywireless communication system.

Suppose h^((t,r))={h₀ ^((t,r)),h₁ ^((t,r)), . . . , h_(W-1) ^((t,r))} isa time-domain channel response vector of length W for the channelbetween the tth transmit antenna and the rth receive antenna. Theaverage power of the coefficient h_(l) ^((t,r)) is expressed by σ₁²=E{|h_(l) ^((t,r))|²} which is independent of the values of t and r.This is because the size of the antenna array in MIMO systems is usuallymuch less than the propagation distance of the first arrival path.

In IEEE 802.11 a/n, a 20-MHz sampling rate is used, resulting in a 50-nstime resolution of the channel response. Normalized power-delay profilecan be expressed as $\begin{matrix}\begin{matrix}{\sigma_{l}^{2} = {G^{- 1}{\mathbb{e}}^{{- l}/{({\Gamma/50})}}}} \\{{= {G^{- 1}{\mathbb{e}}^{{- \alpha} \cdot l}}},}\end{matrix} & \left( {{Equation}\quad 1} \right)\end{matrix}$where α=50/Γ,$G = {{\sum\limits_{l = 0}^{W - 1}{\mathbb{e}}^{{- \alpha} \cdot l}} = {\frac{1 - {\mathbb{e}}^{{- \alpha} \cdot W}}{1 - {\mathbb{e}}^{- \alpha}} \approx \frac{1}{1 - {\mathbb{e}}^{- \alpha}}}}$for αW>>1, and Γ in nanoseconds is the power-delay time constant for thepaths (clusters).

Summing the average power of the coefficients over the delay spread, W,results in ${\sum\limits_{l = 0}^{W - 1}\sigma_{l}^{2}} = 1.$The parameter Γ depends on the propagation distance of the first path(D₀) and path loss model of the channel. To evaluate the average powerof different paths, the propagation distances of these paths should beknown. Because the sampling duration is 50 ns in the foregoing example,the propagation distance between two consecutive paths is 15 meters.Therefore, if D_(l) denotes the propagation distance of the lth path inmeters, D_(l+1)=D_(l)+15 for l=0, 1, . . . , W−2. Without loss ofgenerality, only the power loss ratio of the second path to the firstpath may be considered, which is defined as $\begin{matrix}{R_{loss} = \left\{ \begin{matrix}\left( \frac{D_{0} + 15}{D_{0}} \right)^{2} & {{{if}\quad D_{0}} \leq D_{free}} \\\left( \frac{D_{0} + 15}{D_{0}} \right)^{3.5} & {{{if}\quad D_{0}} > D_{free}}\end{matrix} \right.} & \left( {{Equation}\quad 2} \right)\end{matrix}$where D_(free) space propagation distance. When D₀≦D_(free), the channelis line-of-sight (LOS). Otherwise, the channel is non-LOS. When R_(loss)is given, the parameter α and power-delay time constant Γ can becalculated by solving the equatione^(α)=R_(loss).  (Equation 3)

Assuming D_(free)=15 m, the values of α and Γ are shown in Table 1 forseveral typical values of D₀. The average value of Γ within a room isapproximately 60 ns. TABLE 1 D₀ in meters α Γ in nanoseconds 15 1.38 7045 1.0 50 75 0.64 32 100 0.49 24

Suppose H^((t,r))={H₀ ^((t,r)),H₁ ^((t,r)), . . . , H_(P−1) ^((t,r))} isthe frequency-domain channel response vector of length P for the channelbetween the tth transmit antenna and the rth receive antenna. In otherwords, H^((t,r)) consists of P sub-channels. The Pth sub-channel can berepresented as $\begin{matrix}{{H_{p}^{({t,r})} = {\sum\limits_{l = 0}^{W - 1}{h_{l}^{({t,r})}W_{P}^{l \cdot p}}}},} & \left( {{Equation}\quad 4} \right)\end{matrix}$where W_(P)=e^(−j2π/P). The correlation between the P₁th and P₂thsub-channels is defined asP _(P1,P2) ^((t,r)) =E└H _(P1) ^((t,r))(H _(P2) ^((t,r)))*┘.   (Equation5)

Suppose h_(l) ^((t,r)) is a complex Gaussian variable with zero mean andis independent of h_(m) ^((t,r)) if l≠m. According to Equation 1,$\begin{matrix}\begin{matrix}{\rho_{p_{1},p_{2}}^{({t,r})} = {E\left\lbrack {\left( {\sum\limits_{l = 0}^{W - 1}{h_{l}^{({t,r})}W_{P}^{l \cdot p_{1}}}} \right) \cdot \left( {\sum\limits_{m = 0}^{W - 1}{\left( h_{m}^{({t,r})} \right)^{*}W_{P}^{m \cdot p_{2}}}} \right)} \right\rbrack}} \\{{= {G^{- 1}{\sum\limits_{l = 0}^{W - 1}{{\mathbb{e}}^{{- \alpha}\quad l}W_{P}^{{({p_{1} - p_{2}})}l}}}}},}\end{matrix} & \left( {{Equation}\quad 6} \right)\end{matrix}$which is independent of the values of t and r. Assuming k=P₁P₂ fork=0,1, . . . , P-1 and αW>>1, Equation 6 can be written as$\begin{matrix}{\rho_{k} = {\frac{1 - {\mathbb{e}}^{- \alpha}}{1 - {{\mathbb{e}}^{- \alpha}{\mathbb{e}}^{{- j}\quad 2\pi\quad{k/P}}}}.}} & \left( {{Equation}\quad 7} \right)\end{matrix}$The variable k represents the number of sub-channels spaced between thetwo sub-channels under consideration. From Equation 7, $\begin{matrix}{\left| \rho_{k} \right| = {\frac{1 - {\mathbb{e}}^{- \alpha}}{\sqrt{1 - {2{\mathbb{e}}^{- \alpha}{\cos\left( {2\pi\quad{k/P}} \right)}} + {\mathbb{e}}^{{- 2}\quad\alpha}}}.}} & \left( {{Equation}\quad 8} \right)\end{matrix}$

FIG. 1 shows the curves of |P_(k)| against k for several typical valuesof α when P=256. With the decrease of the parameter α, the correlationbetween the two sub-channels spaced with k sub-carriers is reduced.According to Equation 1, the smaller the parameter α, the morecomparable the average power of the paths. In other words, such achannel consists of more effective multi-paths and therefore the channelbecomes more frequency-selective. In the limit case that a α→0,|P_(k)|→0for any value of k. On the other hand, if the channel is flat fading(non frequency-selective), α→∞, resulting in |P_(k)|=1 for all values ofk.

FIG. 2 shows the curves of |P_(k)| versus k for a different number ofsub-channels P when α=0.64. With the decrease of P, the correlationcurve becomes narrow linearly. For example, the sub-channels with|P_(k)|≧0.9 for P=64 and P=256 have to be spaced less than 4 and 16sub-carriers, respectively.

In order to use the principle of “water-filling”, a measure for CQI mustbe defined. The CQI should be constructed based on the power of thesub-channels. Although |P_(k)| presents the correlation between twosub-channels spaced by k sub-carriers, it does not show clearly thecorrelation of the two sub-channels' power. Therefore, the correlationof sub-channels' power should be derived. The correlation ofsub-channels' power is defined as $\begin{matrix}{\gamma_{p_{1},p_{2}}^{({t,r})} = {\frac{E\left\lbrack \left| H_{p_{1}}^{({t,r})} \middle| {}_{2} \middle| H_{p_{2}}^{({t,r})} \right|^{2} \right\rbrack}{E\left\lbrack \left| H_{p_{1}}^{({t,r})} \right|^{4} \right\rbrack}.}} & \left( {{Equation}\quad 9} \right)\end{matrix}$

With αW>>1, $\begin{matrix}{{{E\left\lbrack \left| H_{p_{1}}^{({t,r})} \right|^{4} \right\rbrack} = {2 - \frac{1 - {\mathbb{e}}^{- \alpha}}{1 + {\mathbb{e}}^{- \alpha}}}},} & \left( {{Equation}\quad 10} \right)\end{matrix}$and $\begin{matrix}\begin{matrix}{{E\quad\left\lbrack {{H_{p_{1}}^{({t,r})}}^{2}{H_{p_{2}}^{({t,r})}}^{2}} \right\rbrack} = {1 - \frac{1 - {\mathbb{e}}^{- \alpha}}{1 + {\mathbb{e}}^{- \alpha}} +}} \\{\frac{\left( {1 - {\mathbb{e}}^{- \alpha}} \right)^{2}}{1 - {2{\mathbb{e}}^{- \alpha}\cos\quad\left( {2\pi\quad{k/P}} \right)} + {\mathbb{e}}^{{- 2}\alpha}},}\end{matrix} & \left( {{Equation}\quad 11} \right)\end{matrix}$where k=P₁-P₂ ε[0,W−1]. Equation 10 and Equation 11 are independent ofthe values of t and r. In the derivation of Equation 10 and Equation 11,it is assumed that the real and imaginary parts of a multi-pathcoefficient, (say h_(P) ^((t,r)) for P ε[0,W-1]), have the same varianceand are independent from each other. Substitution of Equation 10 andEquation 11 into Equation 9 results in $\begin{matrix}{\gamma_{k} = {\frac{\begin{matrix}{{2{{\mathbb{e}}^{- \alpha}\left\lbrack {1 - {2{\mathbb{e}}^{- \alpha}\cos\quad\left( {2\pi\quad{k/P}} \right)} + {\mathbb{e}}^{{- 2}\alpha}} \right\rbrack}} +} \\{\left( {1 - {\mathbb{e}}^{- \alpha}} \right)^{2}\left( {1 + {\mathbb{e}}^{- \alpha}} \right)}\end{matrix}}{\left( {1 + {3{\mathbb{e}}^{- \alpha}}} \right)\left( {1 - {2{\mathbb{e}}^{- \alpha}\cos\quad\left( {2\pi\quad{k/P}} \right)} + {\mathbb{e}}^{{- 2}\alpha}} \right)}.}} & \left( {{Equation}\quad 12} \right)\end{matrix}$

FIG. 3 shows the curves of γ_(k) against k for several typical values ofα when P=256. From FIG. 3, the smallest value of the correlation γ_(k)is around 0.5 at k=P/2. In other words, two sub-channels spaced with P/2sub-carriers may statistically have about 3 dB differences in power.Therefore, it is not necessary to report the CQI for each of thesub-channels. FIG. 4 shows the curves of γ_(k) versus k for a differentnumber of sub-carriers P when α=0.64. The curves are shrunk linearly asthe value of P is reduced.

FIG. 5 is a flow diagram of a process 500 for link adaptation inaccordance with the present invention. Sub-channels are divided into aplurality of groups (step 502). FIG. 6 shows a scheme for generating theCQI in each group of sub-channels. In FIG. 6, the total sub-channels aredivided into Q groups and each group consists of Δ consecutivesub-channels with Δ=P/Q. The correlation of the sub-channels' power in agroup for different values of Q is shown in Table 2. TABLE 2 Statisticaldifferences in power between two sub- The values of Q γ_(k) for 0 ≦ k ≦Δ − 1 channels in a group 20 ≧0.9 0.46 dB 16 ≧0.8 0.97 dB 8 ≧0.6 2.22 dB

A CQI is generated for each group based on channel quality status ineach group (step 504). The channel quality status may be analyzed bydifferent methods including, but not limited to, a signal-to-noise ratio(SNR), a bit error rate (BER), a packet error rate (PER), or the like.Hereinafter, the following embodiment is explained with reference to anSNR. However, it should be understood that other methods may beimplemented alternatively. Assuming that CQI_(q) ^((t)) denotes the qthCQI of the tth transmit antenna (q=0,1, . . . , Q-1 and t=0,1, . . . ,N_(r)−1), CQI_(q) ^((t)) is preferably calculated asCQI _(q) ^((t)) =B+└10 log₁₀(SNR _(q) ^((t)))┘,   (Equation 13)where └x┘ is the largest integer smaller or equal to x, B is an integerwhich should be determined based on system requirements. SNR iscalculated as $\begin{matrix}{{SNR}_{q}^{(t)} = {\frac{1}{\sigma^{2}}{\sum\limits_{r = 0}^{N_{R} - 1}\quad{\sum\limits_{l = 0}^{\Delta - 1}\quad{{H_{l + {q\quad\Delta}}^{({t,r})}}^{2}.}}}}} & \left( {{Equation}\quad 14} \right)\end{matrix}$N_(R) is the number of receive antennas and σ² is the noise variance ineach sub-channel.

The CQI is fed back to adjust communication parameters (step 506). SinceCQI is generated based on the sub-channels in a group, total number ofQ×N_(T) CQIs are generated in a transmission frame (packet), where N_(T)is the number of transmit antennas. It is not necessary to report CQI onan OFDM symbol basis, since the channel may change little in a frame(packet) interval; and due to common phase error (CPE) invoked by thecombination of RF oscillator and the phase-locked loop, the phase of thechannel responses may change. However, such a change does not affect thepower of the sub-channels. Therefore, the CQI can be calculated based onthe channel responses estimated from the long training sequences on aframe basis without using the pilot tones inserted in OFDM symbols. Theinserted pilot tones are used only for the purpose of correcting theCPE.

For example, if each of the CQI indicates one of four states thatcorrespond to the modulation schemes (BPSK, QPSK, 16QAM, 64 QAM), anumber of 2×Q×N_(T) bits are required to report all of the CQIs. In atypical case that Q=16 and N_(T)=4, 2×Q×N_(r)=128 bits are required toreport the CQIs. This is reasonable as compared to the number of data ina transmission frame. Alternatively, the CQI may represent a combinationof two or more communication parameters, such as a combination of acoding rate and a modulation order.

Because any pair of sub-channels statistically has a maximum of 3 dBdifferences in power, the CQI reported according to Equation 13 may bemore meaningful for the change of coding rates rather than modulationschemes. Therefore, the modulation scheme may be kept constant for allthe sub-channels while adjusting the coding rate according to thereported CQI for different groups of the sub-channels. In this case, themodulation scheme may be determined according toM ^((t)) =C+└10 log₁₀(SNR ^((t)))┘,   (Equation 15)where C is an integer which should be determined based on systemrequirements. SNR is determined as follows: $\begin{matrix}{{SNR}^{(t)} = {{\frac{1}{\sigma^{2}}{\sum\limits_{r = 0}^{N_{R} - 1}{H_{0}^{({t,r})}}^{2}}} = {\frac{1}{N_{R}}{\sum\limits_{r = 0}^{N_{R} - 1}\quad{\sum\limits_{l = 0}^{W - 1}\quad{{h_{l}^{({t,r})}}^{2}.}}}}}} & \left( {{Equation}\quad 16} \right)\end{matrix}$

Optionally, after channel estimation, paths with relatively strong powermay be selected. After the selection of the paths having relativelystrong power, the number of effective paths is reduced to M that isusually less than W. Suppose${G_{m^{=}}^{(t)}{\sum\limits_{r = 0}^{N_{R} - 1}{h_{m}^{({t,r})}\quad{for}\quad m}}} \in \left\lbrack {0,{M - 1}} \right\rbrack$is the effective channel response and K is the vector indicating thelocations of the M paths. With G_(m) ^((t)) and K, first all thesub-channels of each antenna can be calculated using Equation 3 and thenthe modulation and coding schemes can be decided for optimization.Optionally the MIMO channel matrix of a reference sub-carrier may betransmitted so that calibration can be made.

Some embodiments for selecting and indexing the reference subcarriersare as follows. In one embodiment, the network configures the referencesubcarriers and the index of the subcarrier(s) are known to both thenetwork and the subscriber. Accordingly, typically, the index of thereference subcarier(s) is not reported to the transmitter. In anotherembodiment, the receiver can dynamically choose reference subcarriersbased on instantaneous channel transfer functions of all subcarriers andother factors in the spectrum. The receiver chooses the index of thereference subcarrier and reports the index to the transmitter.

FIG. 7 is a diagram of a system 700 for link adaptation. The system 700comprises a CQI generator 702 and a link adaptor 704. The CQI generator702 generates a CQI based on channel quality status of received signals706 via each group of sub-channels. A CQI 708 generated by the CQIgenerator 702 is forwarded to the link adaptor 704 for generatingcontrol signals 710 for adjusting communication parameters. Thecommunication parameters include, but are not limited to, a coding rate,a modulation mode, a transmit power level or the like. The link adaptor704 may comprise a look-up table for adjusting communication parametersin accordance with the input CQI. The CQI generator 702 may reside at awireless transmit/receive unit (WTRU), base station or both. The linkadapter may reside at a WTRU, base station or both.

The MIMO-OFDM transmitter and/or receiver of the above embodiments maybe used in a WTRU or base station. The transmitter and/or receiverelements may be implemented as a single integrated circuit (IC),multiple ICs, logical programmable gate array (LPGA), discretecomponents or a combination of any of these IC(s), LPGA, and/or discretecomponents.

A WTRU includes but is not limited to a user equipment, mobile station,fixed or mobile subscriber unit, pager, or any other type of devicecapable of operating in a wireless environment. A base station includesbut is not limited to a Node-B, site controller, access point or anyother type of interfacing device in a wireless environment.

Although the features and elements of the present invention aredescribed in the preferred embodiments in particular combinations, eachfeature or element can be used alone without the other features andelements of the preferred embodiments or in various combinations with orwithout other features and elements of the present invention.

1. A method for adjusting a communication link in an orthogonalfrequency division multiplexing (OFDM) wireless communication system,the method comprising: dividing sub-channels into a plurality of groups;generating a channel quality indicator (CQI) for each group based onchannel quality status in each group; and adjusting communicationparameters in accordance with the CQI.
 2. The method of claim 1 whereinthe CQI is generated based on one of a signal-to-noise (SNR) ratio, abit error rate (BER), and a packet error rate (PER).
 3. The method ofclaim 1 wherein the number of groups is determined in accordance with acorrelation of sub-channels' power.
 4. The method of claim 1 wherein theCQI is calculated based on channel responses estimated from longtraining sequences on a frame basis.
 5. The method of claim 1 wherein acoding rate for each sub-channel is adjusted in accordance with the CQIcorresponding to the sub-channel.
 6. The method of claim 5 wherein amodulation mode for each sub-channel is also adjusted in accordance withthe CQI corresponding to the sub-channel.
 7. The method of claim 1wherein a modulation mode for all sub-channels is adjusted in accordancewith a modulation mode indicator generated based on the entiresub-channels.
 8. The method of claim 1 wherein the CQI represents acombination of two or more communication parameters.
 9. The method ofclaim 1 further comprising a step of selecting a path with relativelystrong power, whereby an effective channel response of the selectedpaths and a vector indicating the location of the selected paths aretransmitted for adjusting the communication parameters.
 10. The methodof claim 1 wherein a multiple-input multiple-output (MIMO) channelmatrix of a reference sub-carrier is transmitted for calibration at atransmitting station.
 11. A system for link adaptation in an orthogonalfrequency division multiplexing (OFDM) wireless communication system,comprising: a CQI generator for generating a channel quality indicator(CQI) for each group of sub-channels based on channel quality status ofeach group, the sub-channels being divided into a plurality of groups;and a link adaptor for adjusting communication parameters in accordancewith the CQI.
 12. The system of claim 11 wherein the CQI for each groupis generated from one of a signal-to-noise (SNR) ratio, a bit error rate(BER) and a packet error rate (PER).
 13. The system of claim 11 whereinthe number of groups is determined in accordance with a correlation ofsub-channels' power.
 14. The system of claim 11 wherein the CQI iscalculated based on channel responses estimated from long trainingsequences on a frame basis.
 15. The system of claim 11 wherein a codingrate for each sub-channel is adjusted in accordance with the CQIcorresponding to the sub-channel.
 16. The system of claim 15 wherein amodulation mode for each sub-channel is also adjusted in accordance withthe CQI corresponding to the sub-channel.
 17. The system of claim 11wherein a modulation mode for all sub-channels is adjusted in accordancewith a modulation mode indicator generated based on the entiresub-channels.
 18. The system of claim 11 wherein the CQI represents acombination of two or more communication parameters.
 19. The system ofclaim 11 wherein the link adaptor comprises a look-up table foradjusting communication parameters in accordance with the CQI.
 20. Thesystem of claim 11 further comprises a means for selecting a path havingrelatively strong power, whereby an effective channel response of theselected path and a vector indicating the location of the selected pathsare transmitted for adjusting the communication parameters.
 21. Thesystem of claim 11 wherein a multiple-input multiple-output (MIMO)channel matrix of a reference sub-carrier is transmitted for acalibration at a transmitting station.
 22. An orthogonal frequencydivision multiplexing (OFDM) wireless transmit/receive unit (WTRU)comprising: a CQI generator for generating a channel quality indicator(CQI) for each group of sub-channels based on channel quality status ofeach group, the sub-channels being divided into a plurality of groups,the CQI being transmitted so that transmission communication parameterscan be adjusted in accordance with the CQI.
 23. The WTRU of claim 22wherein the CQI for each group is generated from one of asignal-to-noise (SNR) ratio, a bit error rate (BER) and a packet errorrate (PER).
 24. The WTRU of claim 22 wherein the number of groups isdetermined in accordance with a correlation of sub-channels' power. 25.The WTRU of claim 22 wherein the CQI is calculated based on channelresponses estimated from long training sequences on a frame basis. 26.The WTRU of claim 22 wherein the CQI represents a combination of two ormore communication parameters.
 27. The WTRU of claim 22 furthercomprises a means for selecting a path having relatively strong power,whereby an effective channel response of the selected path and a vectorindicating the location of the selected paths are transmitted foradjusting the communication parameters.